1. Field of the Invention
The present invention is related to a rotary combustor for burning waste material (e.g., municipal solid waste material), and more particularly, to an axial seal system for defining separate passages for providing combustion air to selected portions along the periphery of the rotary combustor.
2. Description of the Related Art
Numerous types of systems have been developed for solid waste disposal. One type of system for solid waste disposal employs a rotary kiln or combustor formed by a plurality of pipes defining an inner cylindrical surface in which solid waste is burned. The cylinder is rotated about its axis while the solid waste is burned, and the pipes are interconnected to permit a coolant (i.e., water) to flow through the pipes. The water flows through the pipes and is circulated to a heat exchanger for a heat exchange operation, so that the heat created by the burning of the waste materials may be used in generating electricity. Examples of such rotary combustors are disclosed in U.S. Pat. No. 3,822,651 to Harris et al., U S. Pat. No. 4,066,024 to O'Connor and U.S. Pat. No. 4,226,584 to Ishikawa.
FIG. 1A is a schematic plan view of a rotary combustor and FIG. 1B is a schematic cross-sectional view taken along line B--B in FIG. 1A with the outer casing removed and showing connection of the rotary combustor to a heat exchanger and heat exchange fluid supply apparatus. Referring to FIGS. 1A and 1B, a rotary kiln or combustor 20 includes a combustion drum 22 formed by a plurality of tubes 24 which are interconnected to permit the flow of a heat exchange fluid (e.g., water) through the tubes 24. An outer casing 25 surrounds the combustion drum 22. A pair of cylindrical bands 26 are positioned about the periphery of the combustion drum 22 at opposite ends of the combustion drum 22, and the cylindrical bands 26 and 28 are positioned on rollers 30 and 32, respectively. The combustion drum 22 is rotated by any suitable drive arrangement. For example, at least one of the rollers 30 and 32 may be driven by a motor (not shown) to cause the combustion drum 22 to rotate at a relatively slow rate (e.g., in the range of 1.5 to 3 rpm). Alternatively, rollers 30 and 32 may be freely rotating rollers and the combustion drum 22 may be driven through a gear drive arrangement.
Solid waste 34 is fed into a waste receiving end 36 of the combustion drum 22. As the combustion drum 22 is rotated, the waste material 34 travels from the waste receiving end 36 to a waste exit end 38. As the waste material 34 is transported from the waste receiving end 36 to the waste exit end 38, combustion fluid (e.g., air) is provided to the interior of the combustion drum 22 via a combustion fluid supply means 40 to cause burning of the waste material 34. It should be noted that when the rotary combustor 20 is initially started up, an auxiliary fuel is employed to ignite the initial batch of waste material 34. The combustion fluid supply means 40 supplies combustion air under pressure from a blower (not shown) and includes an air duct 42 and three combustion fluid supply zones 44. Each of the combustion fluid supply zones includes control ducts 46 and 48, wherein the control ducts 46 and 48 are employed to supply combustion air to two windboxes (described below) which are included in each of the combustion fluid supply zones 44. The combustion fluid supply zones 44 are separated from each other by division plates 50 to maintain a fluid seal between the combustion fluid supply zones 44.
In order to prevent damage to the rotary combustor 20 due to high temperatures, the combustion drum 22 is cooled by the tubes 24 via a heat exchange fluid which is supplied to the tubes 24 via supply pipes 56 and 57. The supply pipe 57 is coupled to a joint 58 which serves as a rotary coupler for the supply pipe 57, so that heat exchange fluid can be supplied to and from the combustion drum 22 while the combustion drum 22 is being rotated. A pump 60 is connected to the joint 58 via supply pipes 62, and is also connected to a heat exchanger 64 via supply pipes 66. Thus, the heat exchange fluid which is heated by the heat from the burning of the waste material 34, is supplied to the heat exchanger 64 which extracts the heat for purposes of generating electricity, thereby reducing the temperature of the heat exchange fluid before it is returned to the tubes 24 of the combustion drum 22 via the pump 60, the joint 58 and the supply pipes 66, 62, 57 and 56. The heat exchanger 64 may be a steam turbine for generating electricity. At the waste exit end 38 of the combustion drum 22, solid combustion products 52 and exhaust gases 54 are discharged. The heat extracted from the heat exchanger 64 may be supplemented by the heat from the exhaust gases 54 which travel up a flue 68 positioned over the waste exit end 38 of the combustion drum 22.
Referring to FIG. 2 which is a schematic cross-section of FIG. 1A taken along line 2--2, windboxes 70 and 72 which provide 450.degree. F. combustion air to the rotary combustor 20 in one of the combustion fluid supply zones 44, are illustrated. As illustrated in FIG. 2, the combustion drum 22 is a rotatable cylindrical drum which is rotated in the direction of the arrow W in FIG. 2. Further, as the combustion drum 22 is rotated, all of the waste material 34 is shifted to one side of the drum as it travels from the waste receiving end 36 to the waste exit end 38 of the combustion drum 22. The combustion drum 22 is formed by welding perforated steel plates 73 in between the tubes 24. The perforations in the plates 73 allow the combustion air to be blown into the interior of the combustion drum 22. Windbox 70 provides overfire combustion air through the perforations to the interior of the combustion drum 22 in the direction of arrow X in FIG. 2, while windbox 72 provides underfire combustion air through the perforations to the interior of the combustion drum 22 in the direction of arrow Y in FIG. 2. It has been determined that the provision of both underfire and overfire combustion air results in the most complete combustion of the waste material 34. The windboxes 70 and 72 provide separate air passages so that combustion air is provided at predetermined portions along the periphery of the combustion drum 22, as the combustion drum 22 is rotated. The means for defining these passages (by providing a fluid seal at the periphery of the rotating combustion drum 22) includes axial seals 74 which extend from the tubes 24 along the outer periphery of the combustion drum 22. Dividers 76 define the windboxes 70 and 72, and extending from each of the dividers 76 is a T-shaped rigid shoe 78 which is positioned adjacent the periphery of the combustion drum 22, so that the axial seals 74 contact the rigid shoe 78 as the axial seals 74 are rotated past the rigid shoe 78. As a result, gross air seals are provided for the windboxes 70 and 72.
While currently available axial seal systems such as that depicted in FIG. 2, are capable of providing sufficient air seals for the windboxes 70 and 72, these systems require final adjustment of the axial seals 74 in the field and are difficult to assemble and to adjust to provide an adequate seal. That is, the position of each of the axial seals 74 must be adjusted to ensure an adequate seal when the axial seal 74 is rotated past the rigid shoe 78. A typical axial seal 74 will have 15 to 20 nuts and bolts which must be adjusted and tightened in the field. Thus, a typical rotary combustor 20 will have 2,000 or more nuts and bolts to adjust and tighten once the axial seals 74 are positioned in place.
In addition to the above-described problem presented by the required field adjustments, is the related problem of thermal growth of the rotary combustor 20 in the radial direction. That is, because the combustion drum 22 will tend to expand and contract with temperature, the axial seal 74 may not contact the rigid shoe 78 (in which case no seal is provided) or the axial seal 74 may be bent and damaged by the rigid shoe 78 if too large a portion of the axial seal 74 comes in contact with the rigid shoe 78. As a result, proper adjustment of the axial seal 74 requires consideration of the radial expansion and contraction of the combustion drum 22, making the proper adjustment of the axial seal 74 even more difficult. Thus, there is a need in the art for an improved axial seal system which is simple to install and which compensates for thermal expansion and contraction in the radial direction by the combustion drum 22.